In a high frequency communication system representing a cell-phone, a high frequency filter or the like is used to remove undesired signals which are not included in a frequency band to be used for communication. An acoustic wave device having a surface acoustic wave (SAW) element is used for the high frequency filter or the like. The SAW element is an element in which the grating electrode such as an IDT (Interdigital Transducer) is formed on the piezoelectric substrate. There is an element using a SH (Shear Horizontal) wave which is a kind of the surface acoustic wave, as the SAW element.
The SH wave is the surface acoustic wave applying a stress to shear the piezoelectric substrate in a direction which is parallel to a surface of the piezoelectric substrate and orthogonal to a propagation direction of the SH wave. An acoustic velocity of the SH wave is fast, compared with a bulk wave propagating in a solid of the piezoelectric substrate. For this reason, the SH wave propagates on the surface of the piezoelectric substrate while emitting the bulk wave into the piezoelectric substrate. Therefore, in the acoustic wave device using the SH wave, the reduction of a loss has a limit.
For the reduction of the loss of the acoustic wave device using the SH wave, the acoustic velocity of the SH wave is reduced by attaching a slow acoustic velocity material on the piezoelectric substrate. The acoustic velocity of the SH wave is made slower than the bulk wave propagating in the piezoelectric substrate (e.g. a slowest lateral wave of the bulk wave). Thereby, the emission of the bulk wave at the time of the propagation of the SH wave is reduced, and the acoustic wave device using the SH wave can reduce the loss. Thus, the device which reduced the loss is commonly called “love wave type SAW device”.
Patent Document 1 (Japanese Laid-open Patent Publication No. 10-247835) discloses that an Au electrode is formed on a rotated Y-cut X-propagation lithium tantalate (LiTaO3) substrate having a cut angle of 0°, and when “h/λ” changes from 0.04 to 0.08 on the assumption that a pitch of the electrode is “λ” and a film thickness is “h”, the loss is reduced. The Patent Document 1 discloses that, by changing the “h/λ” from 0.04 to 0.08, the acoustic velocity of the SH wave (a leaky wave) is made slower than the acoustic velocity of the slowest lateral wave, and hence the loss is reduced.
Patent Document 2 (Japanese Laid-open Patent Publication No. 2001-77662) discloses that an Au electrode is formed on a rotated Y-cut X-propagation lithium tantalate substrate having a cut angle of 36°, and when the “h/λ” changes from 0 to 0.05, the loss is reduced.
Moreover, in the acoustic wave device having the IDT, there is known a technique that reduces undesired lateral-mode waves. Patent Document 3 (Japanese Laid-open Patent Publication No. 2011-101350) discloses that the undesired lateral-mode waves are reduced by widening the widths of electrode fingers of edge regions with respect to a central region in an overlap region of the IDT. Patent Document 4 (Japanese National Publication of International Patent Application No. 2013-544041) discloses that the undesired lateral-mode waves are reduced by adding dielectric films to electrode fingers of edge regions.
The calculation in the Patent Documents 1 and 2 supposes that the electrodes are uniformly formed on the piezoelectric substrate. That is, each of the electrodes is not the grating electrode. For example, in the Patent Document 2, an electromechanical coupling coefficient and a propagation loss in a case where the “h/λ” changes from 0 to 0.1 are calculated. However, in the calculation, the grating electrode is not used as the electrode. Thus, the range of the “h/λ” in the Patent Documents 1 and 2 does not prescribe a range reducing the loss.